1. Field of Invention
This invention relates to improvements in methods and apparatuses for dynamic information storage or retrieval, and more particularly to improvements in methods and circuitry for detecting electrical resistance in electronic components, especially for detecting open and short faults in magneto-resistive read heads of mass data storage devices, hard disk drives, or the like.
2. Relevant Background
Mass data storage devices include tape drives, as well as hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Applications for hard disk drives are still being developed, and are expected to further increase in the future.
Typically, mass data storage devices include a data transducer, or head, that is used to read data from and write data to a rotating magnetic media, usually in the form of a disk or platter on which a material containing orientable magnetic domains is carried. The present invention pertains especially to magneto-resistive data transducers, or heads, which change in resistivity in the presence of magnetic fields adjacent the disk produced by selectively oriented magnetic domains in the magnetic material on the disk. The typical resistance of a magneto-resistive head is in the range of between about 16 and 150 ohms.
Sometimes, however, the head mechanism experiences faults, the faults of interest herein being open and short faults. Efforts have been made to detect such open and short faults; however, such efforts have required that the read head be biased by electrical current, not voltage, and that both the open and short detection thresholds for head resistance vary over bias level.
What is needed, therefore, is a relatively simple and reliable circuit and method for detecting open and short conditions in a circuit element, such as a magneto-resistive data transducer or head, in which the open and short fault conditions can be detected independently of the head bias for voltage preamplifiers. The simplicity of circuit implementation should result in little or no impact on the thermal noise, power supply noise rejection, read signal path bandwidth, head-to-head switching, or read head circuit performance.
In light of the above, therefore, it is an object of the invention to provide a circuit and method for detecting open and short fault conditions in a circuit element, such as a magneto-resistive data transducer, head, or the like.
One of the advantages of the circuit used in accordance with a preferred embodiment of the invention is that an open or short head condition can be established independently of the bias on the head.
These and other objects, features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims.
Thus, according to a broad aspect of the invention, a circuit is provided for detecting a fault in a magneto-resistive head. The circuit includes a transconductance amplifier having an input across which the head is connected. A circuit is provided for determining a ratio of a current in the head to a variable control current applied to maintain a substantially constant voltage at an output of the transconductance amplifier, the current in the head being a function of the variable control current. A trigger circuit triggers a fault indicating output signal if the ratio falls outside a predetermined range. The circuit for determining the ratio of a current flowing in the head to the variable control current may include first and second current mirrors. The first current mirror mirrors the current flowing in the head, and the second current mirror mirrors the variable control current. The circuit for triggering a fault indicating output signal if the ratio falls outside a predetermined range may include a circuit for triggering an open fault indicating output signal if the ratio exceeds a first predetermined ratio, and a circuit for triggering a short fault indicating output signal if the ratio falls below a second predetermined ratio.
According to another broad aspect of the invention, a second circuit embodiment is provided for detecting a fault in a magneto-resistive head. The second circuit embodiment includes a transconductance amplifier having an input across which the head is connected. Means are provided for determining a ratio of a current in the head to a variable control current applied to maintain a substantially constant voltage at an output of the transconductance amplifier, the current in the head being a function of the variable control current. Means are also provided for triggering a fault indicating output signal if the ratio falls outside a predetermined range. The means for determining a ratio of a current in the head to a variable control current may include means for mirroring the head current to provide a mirrored head current, means for mirroring the variable control current to provide a mirrored variable control current, and means for dividing the mirrored head current by the mirrored variable control current.
According to yet another broad aspect of the invention, a mass data storage device is provided. The mass data storage device includes a magneto-resistive head and a transconductance amplifier having a variable gain and having an input across which the head is connected. A capacitor is provided to receive an output current from the transconductance amplifier. A first variable current source produces a first variable current in the head, and a second variable current source causes a second variable current to flow through a reference current path. The first variable current is a function of the second variable current. A circuit produces a voltage to control the variable gain, based upon the second variable current, and a feedback path varies the second variable current to control the output current from the transconductance amplifier to maintain a substantially constant voltage on the capacitor. A circuit determines a ratio of the first and second currents, and a circuit triggers a fault indicating output signal if the ratio falls outside a predetermined range. The circuit for determining a ratio of the first and second currents may include first and second current mirrors, the first current mirror mirroring the first current, and the second current mirror mirroring the second current. The circuit for triggering a fault indicating output signal if the ratio falls outside a predetermined range may include a circuit to produce an open fault indicating output signal if the ratio exceeds a first predetermined ratio and a circuit for triggering a short fault indicating output signal if the ratio falls below a second predetermined ratio.
According to still another broad aspect of the invention, a method is provided for detecting a fault in a magneto-resistive head for detecting magnetic fields in a data storage device. The method includes determining a ratio of a head current to a variable control current that maintains a constant control voltage in a servo loop that includes the head. The head current is a function of the variable control current. A fault indicating output signal is triggered if the ratio falls outside a predetermined range. Triggering a fault indicating output signal if the ratio falls outside a predetermined range may include triggering an open fault indicating signal if the ratio exceeds a first predetermined ratio and triggering a short fault indicating signal if the ratio falls below a second predetermined ratio. Determining a ratio of a head current to a variable control current may include mirroring the head current to provide a mirrored head current, mirroring the variable control current to provide a mirrored variable control current, and dividing the mirrored head current by the mirrored variable control current.